RX-8 - Violent Shuddering After Sometimes Driving ~30 Minutes

About a month ago, approximately once per week, my 2004 Mazda RX-8 automatic has begun to violently shudder so badly that once on the highway I was towed to a garage after hours but after the tow/wait of 2 hours, it drove fine. The other times I was still in my city and gently got it home where after an hour or so, it drove fine. A few of the incidents featured a brief metallic grinding sound when the shaking began. All times had a foul hot grinding metal smell. The shaking does not occur when car is idling. It happens at all speeds/gears once it starts. One time happened at 100km/hr while other times at 50km/hr. I can drive it for a week without incident but can’t trust it for long distances outside of city for fear of another tow.

One garage didn’t have a clue but cost $42 and 4 hours of my Saturday morning.
Second garage guessed alignment/rims which didn’t solve the problem but cost $927 and 4 days. Subsequent attempts by mechanic to reproduce won’t work yet 2 days after rim/tire replacement, I had it happen again.
Third garage (Mazda) has no idea via phone explanation so I didn’t let them at it yet.

Somebody must have an idea!

The first two mechanics’ diagnostic skills are…questionable, IMHO.

What you have described is a classic symptom of problems in either the torque converter or the transmission itself. In most cases, this is due to owners who did not change the trans fluid every 3 yrs/30k miles.

By this time, your trans fluid should have been changed at least twice, based on elapsed time. Depending on how many miles are on the odometer, it is possible that it should have been changed at least 3 times so far. What is the maintenance history of this transmission?

If you are very lucky, simply doing a trans fluid change immediately, followed by another trans fluid change in a month or so, will resolve the problem. However, it is also possible that things have progressed too far for fluid changes to be effective.

My suggestion is to take the car to an independent transmission shop a.s.a.p. for actual diagnosis. Whatever you do, DO NOT go to AAMCO, Lee Myles, Cottman, Mr. Transmission, or any other chain unless you want to be told that you need a new transmission–whether you really do or not. Those chains are reknowned for overselling, overcharging, and poor workmanship. An indy shop that has been in business for at least 3 years is much more likely to give you an honest diagnosis, good workmanship, and a fair price.

Ask friends, neighbors, co-workers, and relatives for indy transmission shop recommendations, and then take the car to the shop as soon as you possibly can.

Shop 2 was “the” transmission experts in the city. They are indy with a good rep, that’s the crazy thing. No worry about the chains - I’m in Canada and those aren’t in my province. I’ve had the car since 2008 and put less than 20,000km on it but I’ve never had the fluid replaced. I’ll cross my fingers it’s not too late. Thank you for your quick guidance. I’ll update when I’ve been to the shop again.

I own a 2011 Mazda RX-8, and am a member of one of the better RX-8 forums on the internet.

How many miles/kilometers are on your RX-8, and where are you (Canada)?

Is your Check Engine Light lit up in your gauges?
When was the last time the Ignition Coils, spark plug wires, and spark plugs were replaced? The ignition coils on these cars were very weak, and typically only have a 30k mile lifespan before they start causing misfires in the electrical system.

The Mazda Rotary engine is very, very vulnerable to engine misfires, and a weak ignition system. Without a properly operating ignition system, the spark plugs can not ignite the fuel mixture as the rotor spins. The engine already runs very rich from the factory settings, and if not properly ignited, this raw fuel then washes away the protective oil film from the rotor housing, and this leads to rapid wear on the rotor’s Apex Seals (think piston ring seals on a normal piston engine).

The raw fuel also contaminates the catalytic convertor which can lead to that expensive part also being damaged.

Finally, as the apex seals wear away prematurely, the engine loses compression, and misfires become even more common, along with power loss, and driveability issues.

If you live in the US or Canada, Mazda put an 8 year, 100k mile warranty on the RX-8 cars just for the rotary engine. If you are lucky, your warranty hasn’t expired yet, and can get a replacement engine at your local Mazda dealer. You might want to call Mazda’s contact line to find out if you are still within this warranty period. If you are outside of the US and Canada, I’m sorry, but you don’t get this extended warranty on the engine.

If it were my car, I would start with calling Mazda to see if the warranty is still in effect, then progress to replacing the ignition system if it hasn’t been replaced in the past 30k miles, then get a compression test at a Mazda dealer to see the condition of the motor, and then decide if I’m going to pay out of pocket for a replacement motor either at a salvage yard, or a rebuilt unit from a NON-MAZDA source.

Yes, I said a non-Mazda source, if you are paying out of pocket for an engine replacement. you will get a better quality unit 9 times out of 10 in comparison to buying a Mazda rebuilt unit.


Compression test done this year. All good.
Plugs, wires and coils replaced in November.
In New Brunswick.

I would think there would be evidence of metal shavings in the transmission or rear end differential.

Drive a few miles on the highway a few miles using the brakes as little as possible then feel each wheel.
Is one hotter than the others?